Mini-muscles go for a swim

Rat heart muscle cells have been grown on the surface of a polymer, and the resulting thin film can twist, grip and pulse
like a real piece of muscle.

Researchers hope the material may one day be used to make patches to repair a disease-damaged heart, although it may also
find a use in tiny robotic devices.

The thin films were made by Adam Feinberg at Harvard University in Cambridge, Massacheusettes, and his colleagues. They started
with a thin film of plastic (polydimethylsiloxane) and onto which they painted lines of a protein called fibronectin, which
helps with the natural wound-healing process. Heart muscle cells from rats were then seeded onto the plastic, and they grew
along the protein lines in a structured way. "This gets all the muscle cells aligned in the same directions," says Feinberg,
"so they all contract in the same direction."

The resulting film behaves like normal heart muscle fibres, contracting on their own or when prompted by an external electrical
charge.

The films can easily be manipulated and cut into shapes with a scalpel, although they must be kept moist, with the right balance
of electrolytes and nutrients, to survive.

Swimming shapes

Feinberg and Kit Parker, also at Harvard, were initially looking to grow the muscle tissues for therapeutic uses. But Feinberg
spotted the films' potential as micromachines when he noticed how they were bending as the muscle cells contracted. "It was
obvious they had this life-like character," he says.

A rectangular patch of the material bends and unbends with the contractions. As a result, Feinberg has developed a piece of
film that can grip, and one that acts like a motorized spring. A triangular piece, he found, will propel itself forward with
contractions, effectively 'swimming' through solution.

Such shapes could perhaps be used to replicate the swimming motions of extinct creatures, the team suggests, such as the Basilosaurus — a 35-million-year-old whale that swam with the undulating motions of an eel. "We could use the films as a scientific tool
to understand the biomechanics of marine life," says Parker.

Or they could be put to work. "The heart is a machine; if you organize these things properly you can get them to perform work
for you," says Parker. The forces that the artificial muscles exert are comparable to those from real heart muscles, he says.

Alternatively, Parker jokes, the films could be used as fishing bait: "It's a lot easier to get a muscular thin film on a
hook than a worm."

The heart of the matter

The range of movements that can be performed by these films is impressive, says Jonathan Rossiter, who researches biomimetics
at the University of Bristol, UK. Although he thinks practical applications are still a few years away, he thinks that the
films could be very useful. "A number of applications are waiting for this type of 'smart film', although some may seem a
little like science fiction," he says, such as micro-autonomous robots that swim around the blood stream clearing blockages,
or tiny hairs that pulsate and clear phlegm from the airways.

On a larger scale, Rossiter imagines devices that could massage the heart inside a patient at the moment of a heart attack.

Heart muscle tissue has been made before, but to get it working properly is a big challenge. One of the problems has been
making fibres with the ability to contract and flex as strongly as natural muscle tissue.

Parker envisages creating an entire replacement part for the heart, by wrapping the films up in the same way as the muscle
fibres in a ventricle. "When the muscular thin films contract you can mimic the exact biomechanics of the heart," he says.
"We want to be able to build our own ventricles from scratch."